Protein-protein interactions underlying molecular recognition are studied, utilizing monoclonal antibodies (Mabs) specific for the protein hen egg white lysozyme (HEL), a protein which has long served as a prototype for investigating the specificity of immune recognition. We are defining fundamental principles that will allow prediction of function from structure, principles that are critical to such applications as antibody and vaccine design. We are investigating 2 important interrelated applications of this paradigm: (1) Molecular basis of immunological recognition. The X-ray structures of HyHEL-5 Fab complexed with 2 single site-directed mutants of HEL have been solved and refined and a third is in progress. These structures show how single amino acid changes can dramatically alter molecular recognition. Thermodynamic and kinetic measurements indicate complex formation of these high affinity Mabs is enthalpically driven, and site-directed mutations which decrease affinity also decrease the enthalpic change. (2) Cognate antigen presentation by B cells. Experiments with B-cell lymphomas expressing the HyHEL-10 sIg from HyHEL-10 transgenic mice have demonstrated that B-cells can present HEL recognized by their surface Ig up to 4 orders of magnitude more effectively than unrecognized antigen. Depending upon the specificity, anti-HEL antibodies can differentially affect presentation of HEL to T-cells. We are also investigating immunogenicity and protective epitopes in Shigella flexnerii. We have completed a breast cancer project. Two division-competent epithelial cell populations have been identified which are candidate stem-cell and progenitor cells. They are candidate target cells for neoplastic transformation.